Our long-term goal is to elucidate the mechanisms that regulate growth factor signaling during development. Complex regulatory networks help create the correct signaling intensities underlying specific cell fate decisions, and provide the flexibility to allow cell fate decisions to be coordinated and modulated by environmental and physiological cues. Deregulation of growth factor signaling is linked to many/forms of human cancer. We study regulation of signaling by the epidermal growth factor receptor (EGFR), which is the prototypical evolutionary conserved growth factor receptor, and is the receptor implicated in the widest number of human cancers. Regulation of EGFR signaling can be broadly grouped into 3 layers. These include intramolecular inhibition in key components of the signaling pathway; regulation by trans-acting factors within cells responding to the EGFR; and regulation through cell-cell communication and cross-talk with other signaling pathways. We have developed novel methods for identifying mechanisms that regulate EGFR signaling during vulval development in the nematode C. elegans. Precise patterning of the vulva requires EGFR signaling to be subjected to all 3 layers of regulation. Small deviations from this regulation result in quantifiable changes in vulval patterning. In this proposal, we study 3 new regulatory mechanisms. We will use molecular, genetic, and biochemical approaches to: (1) determine how intramolecular inhibition of SOS, a key component of the EGFR pathway, helps regulate EGFR signaling intensity; (2) determine how the CLR-1 receptor protein tyrosine phosphatase inhibits EGFR signaling; and (3) determine how the new locus, ear-1, regulates EGFR signaling during vulval cell fate specification.